US officials probing possibility of planned attack

Vice President Joe Biden, campaigning in Dayton, Ohio, said there is "no place in the civilized world" for the attacks and reinforced Obama's vow to bring the killers to justice.

Biden made no reference to Romney's criticism.

Stevens is the first U.S. ambassador to be killed in an attack since 1979, when Ambassador Adolph Dubs was killed in Afghanistan. The State Department identified one of the other Americans killed Tuesday as Sean Smith, a foreign service information management officer. The identities of the others were being withheld pending notification of next of kin.

The White House said Obama was informed Tuesday night that Stevens was not accounted for and was notified Wednesday morning that he had been killed.

In a written statement earlier Wednesday, Obama called Stevens a "courageous and exemplary representative of the United States."

The four Americans, he said, "exemplified America's commitment to freedom, justice, and partnership with nations and people around the globe."

U.S. officials said some 50 Marines were being sent to Libya to reinforce security at U.S. diplomatic facilities in the aftermath of the attack.

The Marines are members of an elite group known as a Fleet Antiterrorism Security Team, whose role is to respond on short notice to terrorism threats and to reinforce security at U.S. embassies. They operate worldwide.

The officials who disclosed the plan to send the Marines spoke on condition of anonymity because they were not authorized to speak publicly.

Following his Rose Garden remarks, Obama went to the State Department to visit employees and offer support. The White House said Obama wanted to express solidarity with U.S. diplomats and thank them for their service.

The White House also lowered its flag to half staff.

Meanwhile, the U.N. Security Council has a long-scheduled meeting Wednesday morning to discuss Libya and diplomats said the United States is seeking a council statement on the attack. U.N. Undersecretary-General Jeffrey Feltman, a former American diplomat and close friend of Stevens', is scheduled to brief the council on Libya.

Stevens was a career diplomat who spoke Arabic and French and had already served two tours in Libya, including running the office in Benghazi during the revolt against Libyan leader Moammar Gadhafi. He was confirmed as ambassador to Libya by the Senate earlier this year.

His State Department biography, posted on the website of the U.S. Embassy to Libya, says he "considers himself fortunate to participate in this incredible period of change and hope for Libya."

Clinton said Stevens had a "passion for service, for diplomacy and for the Libyan people."

He "risked his own life to lend the Libyan people a helping hand to build the foundation for a new, free nation. He spent every day since helping to finish the work that he started," she said.

Stevens joined the Foreign Service in 1991 and spent his early State Department career at posts in Saudi Arabia, Egypt, Syria and Israel. After working for the Senate Foreign Relations Committee staff for Sen. Richard Lugar, R-Ind., Stevens was posted to Libya as deputy chief of mission.

In that post, Stevens wrote several confidential cables back to Washington, describing Gadhafi's bizarre behavior. During the 2011 revolt against Gadhafi, he was one of the last American diplomats to stay in Tripoli and after the embassy was closed, he was appointed to head the U.S. liaison office to the Transitional National Council.